PARIS, Aug 2 (Reuters) - French police on Friday arrested a Franco-Moroccan man they described as an Islamist militant at Roissy international airport on suspicion that he was part of a conspiracy to carry out attacks in France.

The man, whose name and age were not made public, had just disembarked from a plane arriving from Istanbul after his expulsion by Turkish authorities, the Interior Ministry said in a statement.

The man was being held in police custody pending formal charges. A judge had previously opened a preliminary investigation to determine whether the man was part of a conspiracy to carry out attacks, the statement said.

Suspects in terror-related cases can be detained for up to 96 hours without being formally charged.

The statement did not say where the man had travelled before reaching Turkey, which is a common waystation for European Islamist militants en route to Syria.

Thousands of foreign fighters, many from Western Europe, have joined extremist Islamist groups fighting in Syria and Iraq, according to U.S. and European intelligence estimates.

Fears that militants returning from Syria could carry out attacks in the West were heightened in June when a man who had fought in Syria was arrested on suspicion of killing four people at a Jewish museum in Brussels.

France, which estimates that 800 of its citizens have left to join Islamist groups in Syria, said this month it would ban individuals linked to radical Islamist groups from leaving the country to prevent such attacks.

The Interior Ministry gave no further details. (Reporting by Nicholas Vinocur; Editing by Stephen Powell)